(1) Field of the Invention
The present invention relates to a bright welded seam can of tinplate which is excellent in the appearance of the seam, the corrosion resistance and the adhesion of the coating. More particularly, the present invention relates to a bright welded seam can in which the above-mentioned characteristics are improved by forming an oxide surface layer having a specific thickness and an iron-tin alloy intermediate layer on each of the inner and outer surfaces of the side seam.
(2) Description of the Prior Art
A tin-plated steel sheet, that is, a tinplate sheet, is a material excellent in the corrosion resistance, the coating adhesion and the processability and has been used from old as a can blank. Tinplate cans are ordinarily formed according to a method comprising a can blank into a cylinder and bonding both the side edges of the cylinder by lap seam welding, lock seam welding or combination thereof while using a solder. According to this can forming method, a considerable area is necessary for the seam portion, and the method therefore includes a problem in connection with saving of the resource. Furthermore, in case of food cans, bad influences are brought about by soldering. Therefore, a can forming method not using a solder has been desired in the art.
Instead of soldered cans, seamless cans prepared by draw-ironing have been used in certain fields. However, deformation of the side wall by application of pressure is extreme in seamless cans, and hence, they cannot be conveniently used as vacuum cans, that is, cans of the type where canned contents are subjected to retort sterilization. As another instance that can be used instead of the soldered can, there is known a welded can formed by lap-bonding both the side edges of a can body by welding. The area of the lap seam portion in such welded can is much smaller than in the soldered can and this lap bonding method is advantageous in that a particular adhesive, for example, a solder, need not be used. However, known welded seam cans of tinplate are still insufficient in the appearance of the seam, the corrosion resistance and the coating adhesion.
In the manufacture of welded seam cans, lap portions prepared by forming a can blank into a cylinder are subjected to electric resistance welding by passing the lap portions through between a pair of upper and lower electrode rollers or passing the lap portions through between a pair of upper and lower electrode rollers via an electrode wire. However, when tinplate is used as the can blank, a tin plating layer is fused at the welding step and fused tin is transferred to the electrode member or is scattered in the form of fume, with the result that the protecting effect of the tin plating layer is lost and black or blue iron oxide (probably Fe.sub.3 O.sub.4) is readily formed on the surface. A film of such iron oxide comes to have a thickness of 700 to 3000 A, and such defects as bad appearance of the seam, reduction of the corrosion resistance and insufficient adhesion of the coating result.